The present invention relates to integrated circuit MOSFET dynamic circuits and more particularly, relates to an active refresh circuit for use with such circuits.
A reference which teaches the concept of MOSFET dynamic circuitry in the form of a random access memory is U.S. Pat. No. 4,061,999 issued to Proebsting, et al. on Dec. 6, 1977. This patent which is assigned to the assignee of the present invention is hereby incorporated by reference in its entirety. In these dynamic random access memories a plurality of capacitive memory cells are coupled to a plurality of digit lines arranged in pairs of true and complement digit lines. Dynamic MOS memories are characterized by the use of capacitive storage cells. The memory of the Proebsting patent is also distinguished by the fact that during the reading of information from the memory, the sense amplifiers do not provide a direct path from the positive voltage supply, V.sub.DD, to ground and thereby consume considerably less power than earlier devices. In the above referenced patent, circuitry is provided for precharging the digit lines to the full drain supply voltage so that upon reading of information from the memory the high level cells are automatically refreshed to a potential near the drain supply voltage. It would be desirable to positively or actively pull the digit line which is at a high level after reading up to the full drain supply voltage for best refreshing of the high level cells.
Early circuitry for actively refreshing digit cells to the drain supply potential required cross-coupling between the true digit and complementary digit lines, greatly complicating the layouts of the overall memory chip. It is of course quite apparent that the simplest circuit arrangement available allows the most compact layout which is particularly important in the large memory devices. It is therefore desireable that an active pull up circuit be provided which may be connected at any point along a digit line and need not be cross-coupled to the opposite digit line for proper operation.
The active refresh circuitry taught in U.S. Pat. No. 4,028,557 issued to the present applicant on June 7, 1977, avoids the cross-coupling problems of earlier circuitry. However, the circuit in that patent also provides a DC path from the V.sub.DD supply to ground on the low level digit line for a period of time after sensing. For complete dynamic operation this DC path should be avoided at all times.